\documentclass[10pt]{article} % required
\pagestyle{empty} % required
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{color}
\usepackage[T1]{fontenc}

\begin{document}\definecolor{fgC}{rgb}{0,0,0}\color{fgC}\[\begin{matrix}
 U_t = R(U)  \\ 
 U = \begin{bmatrix}
 v_1 \\ 
 v_2
 \end{bmatrix}\;\;\;
 R(U) = \begin{bmatrix}
 -f v_2 \\ 
 f v_1
 \end{bmatrix} 
\end{matrix}\]\end{document}